LTC3637_15 Datasheet, PDF (12/26 Pages) Linear Technology – 76V, 1A Step-Down Regulator

LTC3637
APPLICATIONS INFORMATION
300 _{VOUT = 5.0V}
I_SET OPEN
L = 5.6µH
1000
200
L = 10µH
100
100
L = 22µH
10
L = 47µH
0
0 10 20 30 40 50 60 70
V_IN INPUT VOLTAGE (V)
3637 F03
Figure 3. Switching Frequency for V_OUT = 5.0V
1
100
1000
PEAK INDUCTOR CURRENT (mA)
3637 F04
Figure 4. Recommended Inductor Values for Maximum Efficiency
it is larger than a minimum value which can be computed
as follows:
L > ^{VIN(MAX) • tON(MIN)} • 1.2
I_PEAK
where V_IN(MAX) is the maximum input supply voltage when
switching is enabled, t_ON(MIN) is 150ns, I_PEAK is the peak
current, and the factor of 1.2 accounts for typical inductor
tolerance and variation over temperature. For applications
that have large input supply transients, the OVLO pin can
be used to disable switching above the maximum operat-
ing voltage, V_IN(MAX), so that the minimum inductor value
is not artificially limited by a transient condition. Inductor
values that violate the above equation will cause the peak
current to overshoot and permanent damage to the part
may occur.
Although the above equation provides the minimum in-
ductor value, higher efficiency is generally achieved with
a larger inductor value, which produces a lower switching
frequency. For a given inductor type, however, as inductance
is increased, DC resistance (DCR) also increases. Higher
DCR translates into higher copper losses and lower current
rating, both of which place an upper limit on the inductance.
The recommended range of inductor values for small sur-
face mount inductors as a function of peak current is shown
in Figure 4. The values in this range are a good compromise
between the trade-offs discussed above. For applications
where board area is not a limiting factor, inductors with
larger cores can be used, which extends the recommended
range of Figure 4 to larger values.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency regulators generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of the more expensive ferrite cores. Actual
core loss is independent of core size for a fixed inductor
value but is very dependent of the inductance selected.
As the inductance increases, core losses decrease. Un-
fortunately, increased inductance requires more turns of
wire and therefore copper losses will increase.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard,” which means
that inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequently output voltage
ripple. Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and do not radiate energy but generally cost more
than powdered iron core inductors with similar charac-
teristics. The choice of which style inductor to use mainly
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